The following relates to the nuclear power reactor arts, nuclear reaction coolant system arts, nuclear power safety arts, and related arts.
Light water nuclear reactors are known for maritime and land based power generation applications and for other applications. In such reactors, a nuclear reactor core comprising a fissile material (for example, 235U) is disposed in a pressure vessel and immersed in primary coolant water. The radioactive core heats the primary coolant in the pressure vessel, and the pressure vessel (or an external pressurizer connected with the pressure vessel by piping) includes suitable devices, such as heaters and spargers, for maintaining the primary coolant at a designed pressure and temperature, e.g. in a subcooled state in typical pressurized water reactor (PWR) designs, or in a pressurized boiling water state in boiling water reactor (BWR) designs. Various vessel penetrations take primary coolant into and out of the pressure vessel.
For example, in some PWR designs primary coolant is passed through large-diameter penetrations to and from an external steam generator to generate steam for driving a turbine to generate electrical power. Alternatively, an integral steam generator is located inside the reactor pressure vessel, which has advantages such as compactness, reduced likelihood of a severe loss of coolant accident (LOCA) event due to the reduced number and/or size of pressure vessel penetrations, retention of the radioactive primary coolant entirely within the reactor pressure vessel, and so forth. Additional smaller diameter vessel penetrations are provided to add primary coolant (i.e., a makeup line) or remove primary coolant (i.e., a letdown line). These lines are typically connected with an external reactor coolant inventory and purification system (RCIPS) that maintains a reservoir of purified primary coolant. Further vessel penetrations may be provided to connect with an external steam generator, an emergency condenser, or for other purposes.
Light water reactors are evaluated to determine their response in the event that a pipe outside of the reactor vessel breaks and a loss of coolant accident (LOCA) occurs. The compact integral reactor design was developed, in part, to minimize the consequence of an external pipe break by eliminating large-diameter piping leading to and from external steam generators. However, integral reactors still utilize small bore connecting piping that transports reactor coolant to and from the reactor vessel. For example, in reactors with an integral pressurizer the reactor vessel has penetrations at the top for pressurizer spray and venting. Some emergency core cooling system (EGGS) designs include piping connecting with an emergency condenser. The vessel also has makeup and letdown penetrations for coolant makeup, letdown, and decay heat removal. These lines run from the vessel to one of two valve rooms where isolation valves act to limit loss of water for breaks down stream of the valve rooms.
This arrangement results in three categories of LOCAs. Type 1 LOCAs result from a leak between the vessel and the valve room. Type 2 LOCAs result from a least at penetrations in the upper vessel. Type 3 LOCAs result from leaks that occur in the valve rooms. Type 2 and Type 3 LOCAs do not drain the reactor water storage tanks RWSTs at the end of the LOCA and result in long term cooling using the water left in the RWSTs. Type 1 LOCAs drain coolant into the refueling cavity, draining the RWSTs.